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Pregnancy is a time of transformation. A body that used to be responsive and lithe suddenly seems capricious and noncompliant. Your bowel, bladder, muscles, joints and emotions may no longer seem to be under voluntary control. Your respiratory system, too, can change as your pregnancy progresses. Your lungs may feel crowded as your abdomen swells and increases the pressure on your diaphragm, the muscle that controls your breathing. This is particularly true in late pregnancy, when most women feel they cannot take a deep breath. However, while mild “air hunger” is normal during pregnancy, severe shortness of breath could signal a serious problem.

Hormones and Hyperventilation

Both estrogen and progesterone levels rise during pregnancy. For 100 years, scientists have known that sex hormones alter the breathing patterns of pregnant women -- usually during the first or second trimester -- presumably by stimulating receptors in the brain’s respiratory centers. This is nature's way of helping you provide more oxygen to your developing baby, but it may make you feel like you can't get enough air. According to a case report in the January 2007 issue of the “Canadian Medical Association Journal,” 60 to 70 percent of women become short of breath during the course of their normal pregnancies due to hormone-induced hyperventilation.

Anemia of Pregnancy

Your requirement for iron rises sharply during pregnancy, and most diets are not sufficient to meet this need. Anemia, usually due to iron deficiency, affects up to 80 percent of pregnant women in some countries, and it is sufficiently common in developed nations to be a significant public health problem. Anemia causes a number of symptoms, including fatigue, weakness, lightheadedness, poor exercise tolerance, rapid heartbeat and shortness of breath. Routine iron supplementation helps reduce the impact of anemia during pregnancy.

Blood Clots

Due to alterations in the proteins that coagulate blood, pregnant women are 5 times more likely to develop blood clots in their legs or lungs than nonpregnant women of equal age. Doctors use the term “venous thromboembolism” to describe abnormal clot formation, which most commonly occurs in the veins of your legs. Approximately half of people who form clots in the veins of their thighs will also develop pulmonary emboli, or clots in their lungs. Pulmonary emboli can trigger several symptoms, including mild to severe shortness of breath, chest pain, fever or rapid heart rate. Large pulmonary emboli can be fatal. Women who develop venous thromboembolism during pregnancy must be treated with anticoagulants, or “blood thinners,” to prevent potentially lethal pulmonary emboli.

Asthma

While pregnancy does not increase your risk for developing asthma, preexisting asthma may get worse during pregnancy, and poorly controlled asthma can lead to severe shortness of breath. This is particularly true if you stop using your asthma medications for fear they will harm your fetus. According to the authors of a study published in the March 2010 issue of "Obstetrics and Gynecology," there is some evidence that the gender of your fetus could influence the severity of asthma during pregnancy, with female fetuses being associated with more severe maternal asthma. The American College of Obstetricians and Gynecologists recommends continuation of asthma treatment throughout pregnancy to protect both mother and fetus.

Uncommon Causes of Breathing Difficulties

A number of less common conditions can cause shortness of breath during pregnancy. Heart failure, which can result from severe anemia, a damaged heart valve or pregnancy-related increases in blood volume, may announce itself with breathing difficulties. Pneumonia, bronchitis or a pneumothorax -- a collapsed lung -- can cause shortness of breath, but these conditions are usually associated with other symptoms, such as cough, fever, chills or chest pain. Shortness of breath during pregnancy is usually due to normal bodily changes, but it may be caused by a potentially life-threatening problem. If you develop breathing difficulties at any time during your pregnancy, contact your doctor at once.

About the Author

Stephen Christensen started writing health-related articles in 1976 and his work has appeared in diverse publications including professional journals, “Birds and Blooms” magazine, poetry anthologies and children's books. He received his medical degree from the University of Utah School of Medicine and completed a three-year residency in family medicine at McKay-Dee Hospital Center in Ogden, Utah.